Instrumentation for monitoring rock movement is a key element to safety and economy in any mining or underground excavation operation. Room closure meters, commonly known as extensometers or convergence meters are conventionally used to provide information on ground conditions and stability so as to check the safety of the excavation and prevent accidents during and after the excavation operations. The information can also be used by scientists for excavation modelling and design, in back-analysis of trial excavations, the verification of engineering design data, and the calibration of numerical models used to predict ground behaviour in other regions of the excavation project.
In unstable areas, however, it is often very difficult for a worker to get in to place the room closure meters without risking great physical danger, and yet it is essential to monitor such areas so as to ascertain the degree of instability and to give an indication of whether access to the area should be restricted or prohibited. Unfortunately, there do not appear to be any instruments available which are capable of remote installation and which will then provide an output signal, representative of actual site conditions, for remote reading and interpretation. All that can generally be done is to make visual observations from a safe distance, something not always possible, to verify ground movement and failure.
Many devices to measure room closure have been described in the literature, for example in U.S. Pat. Nos. 3,042,912; 3,786,503 and 4,514,905, but all require on-site installation. Remote installation of a yieldable mine post system is also known and is described in U.S. Pat. No. 5,215,411, but this does not make any measurements and is not, therefore suitable for the present purposes.